CMT 64Kb RAM module for HP-71b question

[mfleming]

(10-11-2021 08:03 PM)J-F Garnier Wrote:  Ok, I may have the beginning of an explanation.
It seems to me that once the Multimod configures a 16K ROM at 80000, then this ROM is not correctly unconfigured at the next configuration, and interfere with the configuration that uses the absolute address 80000 for RAM/IRAM check..
After this incorrect configuration, the 16K ROM is no more at 80000 and next config will pass correctly (putting the 16K ROM again at 80000), the next one will fail again. And so forth.

A Multimod port becomes configured at 80000 if:
- it is a 16K ROM (32K ROM are configured from D0000 downward),
- addresses 3xxxx to 7xxxx are occupied by other MAIN RAM (or other 16K ROM).

Mark may confirm if there is a problem with port unconfiguration (RESET opcode).

J-F

Mapping of a 71B address to the PIC program flash memory occurs with the CONFIGURE bus command while Daisy-In is high. A mapping table is used, and a non-zero value in the table means the address is to a valid flash memory address. The table is cleared whenever the Daisy-In goes high so that no address is valid at the beginning of a configuration pass. However (wait for it) the table is not cleared by a RESETX command.

When does the RESETX command occur? If at the very beginning of the power on sequence, then MultiMod would map its ROMs to the address space last assigned to it until port 5 Daisy-In is raised. Since according to Dave Frederickson's link, RAM is configured before ROM, there would be a clash at power-up, but not during normal operation after MultiMod ROMs are assigned new addresses. Nice sleuthing work, even though I don't fully understand the failure mechanism. Is there another configuration pass following the FREE or CLAIM command?

Looks like a call to the initialize mapping table routine is needed for RESETX. Would it also be necessary to do the same with the UNCONFIG command? I've only seen that in the first configuration pass as some sort address decoding test.

~Mark